Scanning electron microscope is widely used to observe a specimen to get an image in the micrometer or nanometer dimension by detecting signal electrons generated by a primary beam focused on the specimen. Compared with high landing energy, scanning electron microscope with low landing energy (<3 keV) is more popular to observe a nonmetallic specimen such as biological or semi-conductive specimen, because it reduces the radiation damage of the primary beam and the electron charging effect on specimen surface.
At the low landing energy condition, the observed specimen is generally of non-conductive materials, the primary beam current is usually very small such as several hundred picoamperes, even several picoamperes to prevent excessive charging or damage on the specimen. Therefore, the signal electrons excited from specimen which is related to primary beam are very few. From this aspect, the detector needs to collect the emitted signal electrons as many as possible. Additionally, as the signal beam collection efficiency has an influence on the speed of image generation, i.e., the throughput of scanning electron microscope, increasing the collection efficiency of signal electrons is important to the low voltage scanning electron microscope.
On the other hand, signal electrons need to be detected separately. When the primary beam impinges on the specimen, secondary electrons (SEs) with energy of less than 50 eV and backscattered electrons (BSEs) with energy similar to the primary beam are generated. The SE signal reflects the topography information of specimen. The BSE signal mainly reflects information of the material of the specimen, because BSEs are related to the atomic number of observed material. Some BSEs emitted in a small angle with respect to the specimen surface are influenced by fluctuation of the specimen surface, and thus can also reflect the topography information of specimen. Especially for biology specimen which is dyeing by heavy metal, a single BSE image is easy to show the structure of the specimen. Collecting SEs and BSEs separately will help making a better contrast image to distinguish the structure of specimen.
At the low landing energy condition, for any one of the scanning electron microscopes, there is no solution to simultaneously meet the high resolution, the 100% or close to 100% collection efficiency, flexible detection of BSEs with different emitted angles and flexible control of reception of the required type of signal electrons.